It is desirable to reduce the size of an implantable cardioverter/defibrillator (ICD) in order to improve patient comfort, reduce risk of erosion through the skin, and facilitate pectoral placement. Because the batteries and capacitors account for a large portion of the defibrillator, reducing the defibrillation threshold (DFT), or the amount of energy, voltage, or current required to defibrillate the heart, is key to allowing the device size to be reduced. Using less energy to defibrillate has the added benefit of improving patient comfort and reducing trauma to the patient's cardiac conduction system, as well as prolonging battery and device life.
Many techniques have been used in the past to reduce defibrillation thresholds. These include the use of modified electrodes, described by Ideker et al. in "Current Concepts for Selecting the Location, Size and Shape of Defibrillation Electrodes," PACE 1991, 14:227-240 and by Lang et al. in "Implantable Cardioverter Defibrillator Lead Technology: Improved Performance and Lower Defibrillation Thresholds," PACE 1995, 18:548-559, and the use of biphasic waveforms, described by Fain et al. in "Improved Internal Defibrillation Efficacy with a Biphasic Waveform," American Heart Journal 1989, 117:358-364.
Right ventricular (RV) and superior vena cava (SVC) transvenous electrodes are situated in blood, which has nearly three times the conductivity of cardiac muscle. In a discussion of current shunting by the blood during defibrillation between RV and SVC leads in "FEA of Cardiac Defibrillation Current Distribution," I.E.E.E. Biomed. Trans., Vol. 37, No. 4, April 1990 by N. G. Sepulveda, the amount of current shunted is approximated from finite element analysis studies to be nearly 50 percent. By directing the current directly through the heart wall without having it first pass through the blood, current shunting through the blood would be reduced. Also, because most of the potential drop occurs in the "near field" of the electrode, the closer an electrode is to the endocardium and the more surface of the electrode in contact with the endocardium, the more likely that the heart will see a higher potential gradient. Therefore, it is desirable to create a large shadow area and to substantially limit energy shunting through the blood pool during defibrillation to lower the DFT, while maintaining normal blood flow and normal heart motion when no therapy is being delivered.
Deployable defibrillation electrodes that are low profile during introduction into the heart, but that expand to form a relatively high surface area, have been described, such as in U.S. Pat. No. 5,010,894 to Edhag. The electrode head of this defibrillation electrode is formed by a plurality of outwardly-projecting, precurved flexible conductors, which serve as defibrillation surfaces. The proximal ends of the conductors are anchored adjacently in a common connection device at the same time as their distal ends are adjacently anchored to a second common connection device. Before the lead is introduced into a heart via a vein, the electrode head is stretched using a stylet so that the conductors are brought close to each other, thereby giving the electrode head a diameter which is only slightly larger than the diameter of the lead body. After the electrode head has been advanced into the heart, the stylet can be withdrawn, thereby permitting the conductors to expand laterally so as to resiliently press against the surrounding wall along a substantial portion of their length. Current applied through this defibrillation electrode can be evenly distributed to these conductors, which jointly form a relatively large defibrillation area. This can prevent burn damage to the surrounding heart wall. Low defibrillation thresholds can be achieved because the conductors can be evenly distributed inside the heart. Such a relatively large electrode head, however, can impede the flow of blood in the heart.
In U.S. Pat. No. 5,411,546 to Bowald et al., which is incorporated herein by reference in its entirety, a defibrillation electrode is in a nonexpanded state during implantation, and is radially expanded once in its desired permanent vascular location to conform to the walls of the blood vessel. Means for providing expandability are disclosed, including forming the electrode from a shape-memory metal which can be given a shape at a first temperature suitable for implantation, and assume a cylindrical shape at a second temperature, preferably at body temperature.
In U.S. Pat. No. 5,423,864 to Ljungstroem, which is incorporated herein by reference in its entirety, a defibrillation system includes a defibrillation electrode for intracardiac placement, which contains a flexible electrode cable with at least one elongate, electrically insulated conductor and at least one defibrillation surface disposed at the distal end of the electrode cable for delivering defibrillation pulses to the heart. The system further includes control components and circuitry for determining when defibrillation therapy is to be administered. By providing a large surface area intracardiac defibrillation electrode, current can be distributed so as to prevent damage to the heart. To provide such an electrode without impeding the blood flow during periods when it is not used to emit pulses, the electrode head is constructed so as to be expandable, and the defibrillator housing includes control elements and circuitry operable on the electrode head via the electrode cable to cause the electrode head to expand as needed, and to subsequently return to a non-expanded state. The two means disclosed for expanding the electrode head include a pump which pumps fluid through a channel in the electrode cable to and from a balloon, and a motorized spool from which a line or thread can be wound and unwound to collapse and expand leg components of an electrode. Presumably due to the large amount of energy expended in pumping the fluid through the channel, the electrode is left deployed until sinus rhythm is redetected. This may not be desirable in some cases, such as during the administration of CPR, since the expanded device would impede blood flow. Another drawback to this system is that a shock is delivered only after the expansion logic determines that the electrode head has been expanded; in the event that the expansion mechanism fails, no shock would be delivered. Another drawback to this system is that were the device to fail in the deployed position, blood flow would be seriously impeded. Still another drawback to the system in which a balloon is pumped with fluid is the possibility of hydrodynamic shock, due to the fast change of volume within the heart.